Chair back



FIG. 1 is front elevational view of a chair back showing our new design;

FIG. 2 is a rear elevational view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a bottom plan view thereof; and,

FIG. 5 is a left side elevational view, the right side being a mirror image thereof.

The broken lines shown in FIGS. 2 and 4 are for illustrative purposes only and form no part of the claimed design. 

The ornamental design for a chair back, as shown and described. 